Since the invention of the gas effusion separator in the 1960's by Watson and Biemann and its improvement, the jet separator, invented by Ryhage, it has been possible to efficiently remove carrier gases from the flow of gaseous molecules exiting the end of a Gas Chromatography (GC) column. The gases commonly used in the GC experiment include Helium, Hydrogen, and Nitrogen. In all cases described in the literature the species passing through the jet separator are present as neutral atoms and molecules. The molecules exiting from the jet separator directly enter into the mass spectrometer (MS) where they are ionized in an ionization source, which is operating under high vacuum conditions. The prime function of the jet separator used in GC/MS is to remove the carrier gas while enriching the flow of neutral molecules of analyte molecules into the mass spectrometer.
In contrast to the GC instrument, an atmospheric pressure ionization (API) instrument generates ions external to a mass spectrometer high vacuum system. This being the case, the majority of API source MS instruments generate ions in the presence of an electrical field. This electric field is also used to direct the ions formed during the ionization process towards the inlet of the MS. In desorption electrospray ionization (DESI) and other desorption ionization techniques, the generation of ions at atmospheric pressure can be accomplished with the sample at ground potential. For example, there is often no component of the system to which an electrical potential can be applied in order to selectively focus ions towards the mass spectrometer inlet. In these circumstances, the transfer of ions into the inlet of the MS relies in large part on the action of the vacuum to draw the ions into the MS inlet. MS sources often contain multiple pumping stages separated by small orifices, which serve to reduce the gas pressure along the path that the ions of interest travel to an acceptable level for mass analysis; these orifices also operate as ion focusing lenses when electrical potential is applied to their surface.
A desorption ionization source allowing desorption and ionization of molecules from surfaces, ionization direct from liquids and ionization of molecules in vapor was recently developed by Cody et al. This method utilizes low mass atoms or molecules including Helium, Nitrogen and other gases that can be present as long lived metastables as a carrier gas. These carrier gas species are present in high abundance in the atmosphere where the ionization occurs.
While this ionization method offers a number of advantages for rapid analysis of analyte samples, there remain encumbrances to the employment of this technique for a variety of samples and various experimental circumstances. For example, it would be advantageous to increase the sensitivity of the desorption ionization technique by improving the transfer efficiency of sample related ions from their point of generation to the mass analyzer of the mass spectrometer. Further, it would be desirable to be able to direct the desorption ionization source at an analyte sample at a significant distance from the mass spectrometer. In addition, desorption ionization would have more impact if it was possible to utilize the technique on conventional high vacuum ionization sources encountered in most mass spectrometers.